Take a look at the map at
galtafell.com/index.php
There are a few small stores within the area bounded by Njarargata Sklavrustgur Bankastrti Lkjargata Frkirkjuvegur Sleyjargata one of which, called Kramb, is on the corner of Sklavrustgur opposite Hallgrmskirkju.
There is a Bnus supermarket in Laugavegur, between Frakkastgur and Vitastgur. There is a 10-11 supermarket in Austurstrti, between Psthsstrti and Lkjargata.
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Forgiveness does not always lead to a healed relationship. Some people are not capable of love, and it might be wise to let them go along with your anger. Wish them well, and let them go their way.